Digitization of voice and images and an increase in the quality thereof have advanced with the development of an information-oriented society these days, and the amount of data communication over the internet has been increasing remarkably. With this increase, the amount of electronic data stored in servers, etc. increases, so that there has been a need for the capacity of the information recording system to be increased. Increasing the recording density is required of an optical disk and a magnetic disk mounted in a computer, etc. in order to store huge information without enlarging the device as one of the information recording devices. Increasing the recording density means making the recording bit size minute.
In an optical disk, a method is used where the beam spot of a laser beam is focused to a bit size using a lens. Making the laser beam a shorter wavelength is effective for making the spot size minute. The minimum spot diameter of beam focused by a lens is expressed in terms of the ratio of the wavelength and the numerical aperture of the lens used for focusing, and the shorter the wavelength, the more effective is the increase in the recording density. However, there is a limit to the increase in the density using a technique where the spot is made smaller by making the wavelength of the laser beam shorter. Therefore, a smaller beam spot is necessary for a bit size which is required of a recoding density of the Tb/in2 class. In order to solve this problem, it has been discussed that the spot size is made smaller not by focusing beam using a lens, but by narrowing the distance between the recording medium and the head and using optical near-field.
In order to achieve a high recording density in a magnetic disk device, it is necessary that the distance between the recording medium and the head be made smaller and the crystal grain size of the magnetic film of the magnetic recording medium be minute. Making the crystal grain size in the magnetic recording medium minute is attended by the problem that the grains become thermally unstable. In order to secure the thermal stability while the crystal grain size is made minute, it is effective to make the coercivity greater. An increase in the head magnetic field strength necessary for recording is needed because of an increase in the coercivity. However, since there is a limitation in the characteristics of a magnetic pole material used for a recording head and a limitation in making the distance between the magnetic disk and the head smaller, it is difficult to increase the coercivity with an increase in the recording density. In order to solve the aforementioned problem, a hybrid recording technology has been proposed in which an optical recording technology and a magnetic recording technology are combined. The coercivity of the medium is decreased by heating the medium concurrently with generating an applied magnetic field when recording. As a result, recording becomes easier on a recording medium with high coercivity where recording has been difficult with a conventional magnetic head because of insufficient recording magnetic field strength. A magnetoresistance effect used for a conventional magnetic recording is used for reading. This hybrid recording method is called thermally assisted magnetic recording or optically assisted magnetic recording. Herein, as a heating mechanism by using a beam, a means where a laser beam is focused by using a lens can be used, which is used in conventional optical recording. However, the increase in the recording density of the magnetic disk device also has a limitation in the spot diameter which can be focused by a conventional technique, the same as the optical disk. As a means for solving this, a means utilizing optical near-field has been proposed, the same as the optical disk.
In an optical recording and thermally assisted magnetic recording which uses optical near-field, a laser beam generated by a laser source is guided to the recording head and the spot radius is converted to a suitable size and shape for recording by using an element which has a function of generating optical near-field (hereinafter, it is called an optical transducer) Generally, a semiconductor laser diode (hereinafter, it is called LD) which is small and low power consumption in the laser source is used for a laser source because of the necessity to use it in a disk drive package.
Both optical recording and thermally assisted magnetic recording need sufficient beam intensity for recording. In an optical recording, it is a beam intensity which is necessary for changing the characteristics of the material constituting the bits, and, in a thermally assisted magnetic recording, it is a beam intensity which is necessary for heating it up in order to sufficiently decrease the coercivity for easily performing the magnetization reversal of the recording medium.
The optical output generated by a semiconductor laser is generally about 30 to 100 mW in the 780 nm wavelength range and the 650 nm wavelength range which are the wavelength ranges spread widely as a present light source for optical recording. Optical loss is produced until the optical output reaches the surface of the recording medium and it becomes about several mw. The same level of optical output is required at the surface of the recording medium even in the applications of a thermally assisted magnetic recording device and an optical recording device which uses optical near-field to achieve a recording density in excess of a Tb/in2.
The optical transducer is an element which generates beam with a very small spot size from beam with a relatively larger spot size by using a plasmon resonance. The size of one bit is 25 nm or less in a recording density of Tb/in2 class and the size of the optical transducer is about several hundreds of nanometers to 1 micrometer. An optical transducer is described in, for instance, JP-A No. 2004-151046.
An optical component is used for guiding the beam generated by an LD element to the optical transducer. Since optical loss is produced until the beam generated by the LD element is guided to the optical transducer and the spot size of the beam focused by a waveguide and a lens is larger than the size of the optical transducer, the beam which enters the optical transducer and is converted to the optical near-field is only about several to ten-odd percent of the injected light. Therefore, concerning the optical loss produced until reaching the recording medium, a sufficient optical output is required for the LD element. However, the beam intensity that a semiconductor LD element can generate is not limitlessness, so that it should be driven by an optical output generated by a certain driven current and a power consumption which are rated for an LD element.
An optical component which guides a laser beam generated by an LD element to the optical transducer is a reflector, a lens, and an optical waveguide. The beam generated by the LD element propagates in the optical component arranged in the optical path and reaches the optical transducer and the recording medium ahead thereof. The beam intensity attenuates during transmission through the optical path and it becomes a fraction of several to several tenths the optical output generated by the LD element. The main reason of the attenuation of the beam intensity is adsorption loss and scattering loss during propagation through the optical components and coupling loss caused by the misalignment (displacement of the optical axis and difference in the spot size) created by connections among the optical components. These optical losses are collectively called propagation losses.
In order to obtain sufficient beam intensity necessary for recording, it is necessary to increase the beam intensity generated by the LD element or to decrease the propagation loss. Driving the LD element by a large current is necessary to increase the optical output of the LD element and an increase in the output of the LD element is necessary. However, since there is a limit to the optical intensity generated by the LD element, it is not realistic to enlarge only the optical intensity of LD element. Because making the output of the LD element greater is generally attended by enlargement of the element. The enlargement thereof requires operation using large currents and this causes a remarkable increase in the power consumption and generation of heat in the LD element. Therefore, the key technology is to effectively guide the beam generated by the semiconductor laser to the tip of the head, that is, by decreasing the propagation loss.
In order to efficiently guide the light generated by the LD element to the tip of the head, it is a requirement that the optical loss produced inside the optical components and the coupling loss produced at the junctions of the optical components be made smaller. The optical loss produced in the optical components arises from the adsorption and scattering of the beam caused by the characteristics of the material constituting the components. The coupling loss produced at the junction of the optical components is mainly caused by the misalignment of the shape of the optical field that each optical component has, the distance in the direction of the optical axis which is created while the optical components are connected, and displacement of the components in the direction perpendicular to the optical axis. It is necessary for decreasing the former optical loss to decrease the optical distance by using a material with low scattering and low adsorption and making the components smaller. And it is necessary for decreasing the later coupling loss to match the size and shape of the optical field that the components have and to prevent the optical axis from being displaced in the axial direction and the horizontal direction of the components. When the optical loss is decreased, the optical output required for the LD element can be reduced, resulting in the optical output necessary for recording being obtained. Moreover, the LD element can be made smaller and a lowering of the power consumption can be achieved. As a result, the limitation caused by the size of the LD element becomes smaller and the freedom of design thereof, such as the arrangement of the LD element, etc., increases.
The spot size of the laser beam emitted from the LD element is generally about 1 μm to 2 μm at the emission face and, it is too large to be applied the recording bit as is since it enlarges with distance from the emission face. In order to convert the spot size and the shape, an optical transducer is necessary. It is difficult to completely eliminate the occurrence of the propagation loss so long as optical components are used, from the LD element to the optical transducer, and there is an actual limitation on decreasing optical loss only by material selection and alignment technique. Moreover, the optical recording head and the magnetic recording head itself are smaller than these optical components and, when the optical components are arranged, there is a limitation on mounting caused by the size of the components themselves and the tolerance required for the optical system, so that a structure where the propagation loss is reduced is difficult to achieve.
Therefore, decreasing the number of optical components is the best solution for solving the limitation on mounting as well as for achieving the decrease in the propagation loss produced between the light source and the recording medium. Therefore, it is preferable that the LD element which is a light source be arranged in the vicinity of the head and be connected to the head part without using any optical component between the LD element and the head. That is, integrating the LD elements over the head is one of techniques for minimizing the propagation loss.
However, especially in a magnetic disk typified by a hard disk drive, the size of the tip part which is a main component of the recording head is a hexahedron having one edge of several hundred micrometers and the size is on the same order as the LD element. Therefore, a limitation exists on mounting caused by the size of the LD element in the case of the arrangement where the LD element which is a light source is mounted over the head.
In the optical disk head and the thermally assisted magnetic recording head which use the optical near-field, a means is adopted in which the LD element is installed outside of the head and the beam is guided to the optical transducer provided at the tip of the head by using the optical components in the example where a structure is realized in the experiment. As shown, for instance, in Japan Journal of Applied Physics, Vol. 42 (2003), pp. 5102 and Optics Letters, Vol. 25 (2000), pp. 1279), a means is adopted in which the LD element is arranged at a position except for the head and the beam is guided to the recording head by using an optical waveguide such as an optical fiber, etc. However, in a case where a practical recording head is concerned, it is considered that the propagation loss becomes greater when the LD element which is a light source is separated from the head and the beam passes through a plurality of optical components.